YEP- I occasionally get the very same itchy heat-rash looking rash on the tops of my thighs underneath my shorts too. Same thing!

I came to the conclusion it was a reaction produced somehow by the sun hitting lycra material.
One morning we were having pre-ride breakfast in our outdoor cafe, and I was sitting in the sun. The morning was still pleasantly in the 70's, and I hadn't exerted myself yet. After a while I felt my legs itching under the shorts and went in the ladies room and sure enough- the rash was starting! Only under the shorts, on the tops of my thighs only. It had been activated merely by my sitting in the sun in my shorts....not really so much from being overheated or sweaty. I put on some cool feeling hand cream, sat with my legs in the shade, and it went away.

When this rash is mild, it can go away in a few hours. When it's bad, it stays for a day or two. I found it did help to put sunblock or other lotion on my legs there before putting on my shorts.